The mechanism of Ca++ translocation in sarcoplasmic reticulum was investigated using fast kinetic methods. Results of these studies indicate that the translocation mechanism involves rapid transfer of Ca++ from an external binding site to an occluded site prior to its release at the inner membrane surface. The kinetics of the phosphoenzyme intermediate reactions of the electric organ (Na,K)-ATPase were compared in the presence and absence of K+. Results indicate the presence of K+-activated acceleration of the phosphoenzyme interconversion reaction favoring a mechanism involving simultaneous occupation of Na+ and K+ transport sites. Examination of the initial turnover characteristics of the purified kidney (Na,K)-ATPase provides additional support for a simultaneous transport mechanism. The enzymatic partial reactions of the cardiac myofibrillar ATPase were examined with respect to Ca++ sensitivity. Preliminary results indicate the presence of regulation of the steady state rate involving Ca++ activated release of reaction products.